The present applicant has previously disclosed an endoscope objective lens having a four-group, six lens element construction in Japanese Laid-Open Patent Application S63-261213. The present applicant has also previously disclosed an endoscope objective lens having a four-group, five lens element construction in Japanese Patent Application 2002-218696. The endoscope objective lens disclosed in Japanese Patent Application 2002-218696 is intended to provide a wide-angle view with an adequate back focal length while correcting lateral color, results which are difficult to satisfy simultaneously.
Enlarging the field angle of a lens in order to create a wider angle lens requires shortening the focal length if the image size remains constant. However, if the size of the entire lens system is simply reduced proportionally based on an existing endoscope objective lens, the back focal length is also proportionally shortened, making it difficult to insert a prism for deflecting the light path between the prism entrance surface and an imaging element. Thus, it is necessary to not shorten the back focal length while achieving wide-angle conversion. However, if the construction is such that the object side lens component is formed of cemented lens elements and operates to correct lateral color, the longer the back focal length, the weaker the correction of lateral color. Because lateral color not only generates color blurring of the image periphery but also reduces the resolution of the peripheral section, a large lateral color aberration prevents precise observation essential to a precise diagnosis based on imaging through the endoscope.
The four-group, five lens element endoscope objective lens mentioned previously provides a good balance of sufficient back focal length, wide-angle view, and correction of lateral color. However, there is a need for an endoscope objective lens with an even longer back focal length in order to assure an adequate back focal length with sufficient space for filters, such as a lowpass filter and/or an infrared rejecting filter, along with a prism between the image side lens element and the imaging element. With the use of such filters, more than five lens elements may be necessary in order to achieve the desired correction of lateral color because of the longer back focal length.